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United States Patent |
5,283,525
|
Lamerichs
,   et al.
|
February 1, 1994
|
Method and device for the volume-selective extraction of a magnetic
resonance spectrum by Hartmann-Hahn transfer
Abstract
An object is placed within the field of magnetic coils which generate a
steady uniform magnetic field. The object contains nuclear spins of
coupled first and second types of nucleus, for example, CH.sub.n in which
C is a .sup.13 C carbon isotope and H is a .sup.1 H proton. Magnetic
resonance signals are generated in the object by RF pulses. Pulse and
gradient sequences are generated for providing magnetization transfer from
the first nucleus to the second nucleus via first and second channels. A
display displays spectra formed from resonance signals processed by a
programmed processor via Fourier tranformation of sampled values. A pick
up coil is tuned to receive the signals from a corresponding nucleus which
is processed by a corresponding channel. The process is volume selective
combined with a Hartmann-Hahn transfer sequence.
Inventors:
|
Lamerichs; Rudolf M. J. N. (Eindhoven, NL);
Van Stapele; Reurt P. (Eindhoven, NL);
Den Hollander; Jan A. (Eindhoven, NL)
|
Assignee:
|
U.S. Philips Corporation (New York, NY)
|
Appl. No.:
|
871947 |
Filed:
|
April 22, 1992 |
Foreign Application Priority Data
| May 23, 1991[EP] | 91201228.3 |
Current U.S. Class: |
324/307; 324/309 |
Intern'l Class: |
G01V 003/00 |
Field of Search: |
324/300,307,309,313,314
|
References Cited
U.S. Patent Documents
4682107 | Jul., 1987 | Bendall et al. | 324/307.
|
4922203 | May., 1990 | Sillerud et al. | 324/307.
|
4987369 | Jan., 1991 | Van Stapele et al. | 324/307.
|
Foreign Patent Documents |
0347990 | Jun., 1988 | EP.
| |
Other References
R. R. Ernst et al., "Principles of Nuclear Magnetic Resonance in One and
Two Dimensions", Oxford Scientific Publications, 1986, pp. 185-191.
R. J. Ordidge et al., "Image-Selected in Viro Spectroscopy (ISIS). A New
Technique for Spatially Selective NMR Spectroscopy", Journal of Magnetic
Resonance, vol. 66, pp. 283-194. (Apr. 1985).
|
Primary Examiner: Arana; Louis
Attorney, Agent or Firm: Slobod; Jack D.
Claims
We claim:
1. A magnetic resonance method for the volume-selective extraction of
spectral information from an object under examination containing a first
type of nucleus and a second type of nucleus which is coupled to the first
type of nucleus, said object being arranged in a steady uniform magnetic
field, said method comprising: performing heteronuclear Hartmann-Hahn
magnetization transfer, and extracting spectral information of at least
one of the first and second types of nucleus from transferred
magnetization of the other of the first and second types of nucleus, at
least one of said performing the Hartmann-Hahn transfer and said
extracting spectral information being substantially only with respect to a
selected volume of the object.
2. A magnetic resonance method as claimed in claim 1, wherein the
heteronuclear Hartmann-Hahn transfer is performed by applying an adiabatic
RF electromagnetic pulse to the first type of nucleus in order to bring a
magnetization of the object out of a state of equilibrium and by
subsequently spin-locking the magnetization brought out of a state of
equilibrium by means of an RF electromagnetic spin-lock pulse while at the
same time applying a cross-polarization transfer pulse to the second type
of nucleus.
3. A magnetic resonance method as claimed in claim 2 wherein a first type
of nucleus is a proton, the second type of nucleus being a carbon isotope
enriched in the object, and causing signal acquisition of magnetization
transferred to protons by enriched carbon isotopes.
4. A magnetic resonance method as claimed in claim 2 wherein the first type
of nucleus is a proton and causing signal acquisition of magnetization
transferred by protons to the second type of nucleus.
5. A magnetic resonance method as claimed in claim 4 wherein the second
type of nucleus is a carbon isotope enriched in the object, and causing
signal acquisition of magnetization transferred to protons by enriched
carbon isotopes.
6. A magnetic resonance method as claimed in claim 1, including providing
volume selection with surface coils.
7. A magnetic resonance method as claimed in claim 1, wherein said
performing the Hartmann-Hahn transfer includes applying a volume-selective
pulse and gradient sequence to the first type of nucleus in order to bring
a magnetization of the object volume-selectively out of a state of
equilibrium and subsequently spin-locking the magnetization
volume-selectively brought out of the state of equilibrium by an RF
electromagnetic spin-lock pulse while at the same time applying a
cross-polarization transfer pulse to the second type of nucleus.
8. A magnetic resonance method as claimed in claim 7 wherein the second
type of nucleus is a carbon isotope enriched in the object, and causing
signal acquisition of magnetization transferred to protons by enriched
carbon isotopes.
9. A magnetic resonance method as claimed in claim 7 wherein the first type
of nucleus is a proton and causing signal acquisition of magnetization
transferred by protons to the second type of nucleus.
10. A magnetic resonance method as claimed in claim 9 wherein the second
type of nucleus is a carbon isotope enriched in the object, and causing
signal acquisition of magnetization transferred to protons by enriched
carbon isotopes.
11. A magnetic resonance method as claimed in claim 7 wherein said applying
the volume-selective pulse and gradient sequence includes applying one of
a PRESS sequence, an ISIS sequences or STEAM sequence.
12. A magnetic resonance method as claimed in claim 7 including providing
the volume-selective pulse and gradient sequence as a localized spin-echo
sequence, and applying a phase encoding gradient after the
cross-polarization transfer pulse and before signal acquisition.
13. A magnetic resonance method as claimed in claim 7, including using a
single double-tuned coil for applying pulses to the respective nuclei.
14. A magnetic resonance method as claimed in claim 1 wherein the first
type of nucleus is a proton and causing signal acquisition of
magnetization transferred by protons to the second type of nucleus.
15. A magnetic resonance method as claimed in claim 14 wherein the second
type of nucleus is a carbon isotope enriched in the object, and causing
signal acquisition of magnetization transferred to protons by enriched
carbon isotopes.
16. A magnetic resonance method as claimed in claim 1 wherein the first
type of nucleus is a proton, the second type of nucleus being a carbon
isotope enriched in the object, and causing signal acquisition of
magnetization transferred to protons by enriched carbon isotopes.
17. A magnetic resonance method as claimed in claim 16 including enriching
the carbon isotope by one of injection and oral administration of glucose.
18. A magnetic resonance device for the volume-selective extraction of
spectral information from an object under examination containing a first
type of nucleus and a second type of nucleus which is coupled to the first
type of nucleus, said device comprising: means for generating a steady,
uniform magnetic field in which said object is arranged, transmitter means
for transmitting RF electromagnetic pulses to said object, means for
generating magnetic field gradients superposed on the steady field, and
receiving and processing means for receiving and processing magnetic
resonance signals generated in the object, programmed control means
operative to apply, by way of the transmitter means, RF pulses to the
first and the second type of nucleus, said programmed control means being
operative to cause the performance of Hartmann-Hahn magnetization transfer
between the first and second types of nuclei and the extraction, by way of
the receiving and processing means, of spectral information of at least
one of said first and second types of nucleus from transferred
magnetization of the other of the first and second types of nucleus, at
least one of the performance of Hartmann-Hahn transfer and the extraction
of spectral information being substantially only with respect to a
selected volume of the object.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a magnetic resonance method for the
volume-selective extraction of spectral information from an object
containing a first type of nucleus and a second type of nucleus which is
coupled to the first type of nucleus, the object being arranged in a
steady, uniform magnetic field, the spectral information being extracted
from magnetization transfer from one of the first and second types of
nuclei to the other.
The invention also relates to a magnetic resonance imaging device for the
volume-selective extraction of spectral information from an object
containing a first type of nucleus and a second type of nucleus which is
coupled to the first type of nucleus, which device comprises means for
generating a steady, uniform magnetic field, transmitter means for
transmitting RF electromagnetic pulses to the object arranged in the
steady field, means for generating magnetic field gradients superposed on
the steady field, and receiving and processing means for the magnetic
resonance signals generated in the object, which processing means include
programmed means and are operative to apply, by way of the transmitter
means, pulses to the first and the second type of nucleus so that the
spectral information is extracted from magnetization transfer.
2. Description of the Prior Art
A magnetic resonance imaging method and device of this kind are known from
European Patent Application No. 0 347 990 which corresponds to U.S. Pat.
No. 4,987,369. The cited Application describes a volume-selective
polarization transfer sequence in which volume selection of a part of the
object coincides with a polarization transfer sequence, three pulses of
which are applied to a first type of nucleus via a first channel, while
two pulses thereof, being at least partly coincident with the three
pulses, are applied to a second type of nucleus via a second channel. The
nuclei are, for example proton-coupled carbon atoms such as in a CH.sub.n
system. Volume selection is achieved by application of slice-selective
magnetic field gradients during application of the pulses.
Even though such a known method offers an improvement over, for example
so-called DEPT sequences (Distortionless Enhancement by Polarization
Transfer) preceded by volume selection such as VSE (Volume Selective
Excitation), the known sequence still has drawbacks. This is because the
transfer efficiency is highly dependent on correct adjustment of the
excitation angle of the pulses to protons as well as .sup.13 C in, for
example a CH.sub.n system. Accurate timing of the pulses is also a
critical factor. For such sequences use is often made of a double surface
coil having different RF profiles, so-called B.sub.1 profiles, for the
proton fields and the .sup.13 C fields. The latter makes adjustment
additionally difficult in practice. For in vivo measurements the
efficiency of the magnetization transfer may be further affected by
excitation angle variations due to local B.sub.1 inhomogeneities.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and device which do
not have such drawbacks.
A method in accordance with the invention is characterized in that the
magnetization transfer is realized by heteronuclear Hartmann-Hahn
transfer, the spectral information of at least one type of nucleus being
extracted from transferred magnetization of the other type of nucleus. In
such a volume-selective Hartmann-Hahn sequence localized transverse
magnetization is spin-locked to, for example to the protons and during
spin-locking to the other type of nucleus, for example proton-coupled
.sup.13 C, an RF electromagnetic field is applied. So-called Hartmann-Hahn
transfer or cross-polarization transfer is achieved when the so-called
Hartmann-Hahn condition is satisfied:
.gamma.'.B.sub.1 '=.gamma.".B.sub.1 "
where .gamma.' and .gamma." are the gyromagnetic ratios of the first and
the second type of nucleus, respectively, and B.sub.1 ' and B.sub.1 " are
the RF electromagnetic fields on the first and the second type of nucleus,
respectively. The sequence in accordance with the invention requires a
much simpler adjustment, because only the ratio of the respective RF
electromagnetic fields need be adjusted. As a result, the significance of
local inhomogeneities in the RF electromagnetic fields, as occurring
during in vivo measurements, is far less than in the known method.
It is to be noted that Hartmann-Hahn transfer is described per se in the
book "Principles of Nuclear Magnetic Resonance in One and Two Dimensions",
R. R. Ernst e.a., Oxford Scientific Publications, 1986, pp. 185-191.
Except that the cited book states that the Hartmann-Hahn experiment per se
imposes severe requirements as regards devices to be used for executing
the experiment, it is also noted that other transfer methods are to be
preferred for liquids.
A version of a method in accordance with the invention is characterized in
that the heteronuclear Hartmann-Hahn transfer is realized by applying an
adiabatic RF electromagnetic pulse to the first type of nucleus in order
to bring a magnetization of the object out of a state of equilibrium and
by subsequently spin-locking the magnetization brought out of the state of
equilibrium by means of an RF electromagnetic spin-lock pulse while at the
same time applying a cross-polarization transfer pulse to the second type
of nucleus. Notably when use is made of surface coils, exhibiting a
non-uniform sensitivity pattern, this version offers major advantages
because the excitation angle over the sensitivity range of the coil is
then independent from the non-uniform sensitivity pattern of the coil
within given limits. It is also to be noted that in the case of known
sequences, such as DEPT, the magnetization transfer mechanism is very
susceptible to variations as regards excitation angle of the nuclear
spins; this is not the case in the method in accordance with the
invention.
A further version of a method in accordance with the invention is
characterized in that the volume selection is realized by surface coils.
The Hartmann-Hahn condition will be satisfied only in a limited region.
The location of the localized region can be varied by variation of the
field strength. This version can be of importance notably for the
measurement of metabolites having a comparatively short T.sub.1 or T.sub.2
relaxation time.
Such a volume selection per se is described in U.S. Pat. No. 4,682,107.
Another version of a method in accordance with the invention is
characterized in that the Hartmann-Hahn transfer is realized by applying a
volume-selective pulse and gradient sequence to the first type of nucleus
in order to bring a magnetization of the object volume-selectively out of
a state of equilibrium and by subsequently spin-locking the magnetization
volume-selectively brought out of the state of equilibrium by means of an
RF electromagnetic spin-lock pulse while at the same time applying a
cross-polarization transfer pulse to the second type of nucleus. The
localization is thus improved in comparison with the use of surface coils
for localization. Volume selection can be realized, for example by a
PRESS, ISIS or STEAM sequence, the spin-lock field being applied at the
instant of echo formation.
Another version of a method in accordance with the invention is
characterized in that the volume-selective pulse and gradient sequence is
a localized spin-echo sequence and in that a phase encoding gradient is
applied after the cross-polarization transfer pulse and before signal
acquisition. Via the localized spin-echo sequence, a bar is then selected
as the part of the object and at the instant of echo-formation a spin-lock
field is applied. The bar is sub-divided by the phase encoding and, after
Fourier transformation, a set of spectra is obtained from the bar. The bar
can be chosen so as to extend through a heart of an in vivo animal or
human object.
Another version of a method in accordance with the invention is
characterized in that a single double-tuned coil is used for applying
pulses to the respective nuclei. Because the B.sub.1 profiles of both
fields are then identical, Hartmann-Hahn transfer can be achieved over a
comparatively large volume.
A further version of a method in accordance with the invention is
characterized in that the first type of nucleus is a proton, the second
type of nucleus being a carbon isotope enriched in the object, signal
acquisition of magnetization transferred to protons by enriched carbon
isotopes then taking place. The sequence in accordance with the invention
will generally be used in conjunction with the use of transfer from an
abundantly present nucleus such as a proton nucleus, to a nucleus which
occurs only rarely, utilizing a "gain" factor which is given by the ratio
of the gyromagnetic ratios of the nuclei. In the present example the same
transfer mechanism is used, but measurement takes place via the channel
other than the customary channel, i.e. via the proton channel in the
present example. However, the nucleus which inherently occurs to a lesser
extent in vivo should then be enriched, for example by injection or oral
administration in order to achieve adequate signal strength. For example,
when the non-toxic glucose is applied in vivo as an enrichment of a
.sup.13 C isotope, the metabolism thereof, notably in the brain of the
object, can be traced. The latter offers major advantages with respect to
so-called PET (Positron Emission Tomography) techniques where radioactive
compounds are injected into the object, for example radioactive modified
glucose compounds.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be described in detail hereinafter with reference to a
drawing; therein:
FIG. 1 shows diagrammatically a magnetic resonance device in accordance
with the invention,
FIGS. 2A to 2E show a first version of a pulse and gradient sequence in
accordance with the invention as well as its effects, and
FIG. 3 shows a second version of a pulse and gradient sequence in
accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows diagrammatically a device 1 in accordance with the invention.
The device 1 comprises magnet coils 2 and, in the case of resistive
magnets or superconducting magnets, a DC power supply 3. The magnet coils
2 and the DC power supply 3 constitute means for generating a steady,
uniform magnetic field. When the magnet coils 2 are constructed as
permanent (superconducting) magnets, the DC power supply 3 will be absent.
An object 5 can be arranged within the magnet coils 2. The object 5 may
contain nuclear spins of a first type of nucleus and of a second type of
nucleus which may be coupled to one another, for example in a CH.sub.n
group of a molecule in which C is a .sup.13 C carbon isotope and H is a
.sup.1 H proton. Other types of nuclei are also feasible. The coupling can
then be expressed by a coupling constant J.sub.CH sec.sup.-1. During
operation of the device 1 with the object 5, which may be an in vivo human
or animal object, being arranged within the magnet coils 2, a slight
excess of nuclear spins (of nuclei having a magnetic moment) will be
directed in the direction of the steady field in a state of equilibrium.
From a macroscopic point of view this is to be considered as an
equilibrium magnetization. The device 1 furthermore comprises means 6 for
generating magnetic resonance signals in the body by RF electromagnetic
pulses. The means 6 comprise a first transmitter/receiver channel 7 and a
second transmitter/receiver channel 8. The first transmitter/receiver
channel 7 is coupled, via a directional coupler 9, to a first coil 10 for
the transmission of RF electromagnetic pulses and for the reception of
magnetic resonance signals. The first transmitter/receiver channel
comprises a modulator 11, an amplifier 12, an amplifier 13, a
phase-sensitive detector 14 and an oscillator 15 for generating a carrier
signal. The modulator 11 may be an amplitude and/or frequency and/or phase
modulator and is controlled by a process computer or programmed control
means 16 which is coupled to processing means 17 which include programmed
means 18. A magnetic resonance signal received via the first
transmitter/receiver channel 7 is demodulated in the phase-sensitive
detector 14 and the demodulated signal is applied to an A/D converter 19.
Sampling values 20 can be applied to the processing means 17. The
construction of the second transmitter/receiver channel 8 is identical and
comprises a directional coupler 21, a transmitter/receiver coil 22, a
modulator 23, an amplifier 24, an amplifier 25, a phase-sensitive detector
26, and an oscillator 27. The phase-sensitive detector 26 is coupled to an
A/D converter 28 which itself is coupled to the processing means 17. When
the device 1 is used to generate pulse and gradient sequences for
realizing magnetization transfer from a first type of nucleus to a second
type of nucleus coupled to the first type of nucleus, the oscillator 15
will be adjusted to the spin resonance of the first type of nucleus and
the oscillator 27 will be adjusted to the spin resonance of the second
type of nucleus. The coils 10 and 22 can be replaced by a surface coil 29
which may be a single double-tuned coil tuned to the respective resonant
frequencies of the respective nuclei. In the latter case this coil is
connected to the directional couplers 9 and 21 via a customary frequency
separation filter for separating the respective resonant frequencies of
the two types of nuclei. The coils 10 and 22 may alternatively be formed
by a double concentrically arranged coil which thus enables volume
selection. When use is made of surface coils, such coils will be
displaceable within the device 1. The device 1 also comprises means 30 for
generating magnetic field gradients superposed on the steady field. The
means 30 comprise gradient magnet coils 31 for generating a magnetic field
gradient G.sub.x, gradient magnet coils 32 for generating a magnetic field
gradient G.sub.y, gradient magnet coils 33 for generating a magnetic field
gradient G.sub.z, and a power supply 34 which is controlled by the process
computer 16 and which serves to power the gradient magnet coils 31, 32 and
33 which are individually activatable. In the embodiment shown the
gradient magnet coils 31, 32 and 33 are arranged in space so that the
field direction of the magnetic field gradients G.sub.x, G.sub.y and
G.sub.z coincides with the direction of the steady field and that the
gradient directions extend perpendicular to one another as denoted by
three mutually perpendicular axes x, y and z in FIG. 1. The device 1
furthermore comprises display means for displaying spectra to be formed
from resonance signals received. The programmed means 18 are operative to
determine, for example by way of a Fourier transformation, spectra from
the sampling values 20, for example obtained via the second
transmitter/receiver channel 8.
FIG. 2A shows a first version of a pulse and gradient sequence sq1 in
accordance with the invention as a function of time t, volume selection
being realized by the coils 10 and 22 which are constructed as surface
coils. By way of example it will be assumed that the first type of nucleus
is a bound proton .sup.1 H, coupled to a .sup.13 C carbon isotope. The
coils 10 and 22 may be concentric coils, the coil 10 being coupled to the
transmitter/receiver channel 7 and being tuned to .sup.1 H, while the coil
22 is coupled to the transmitter/receiver channel 8 and tuned to .sup.13
C. In accordance with the invention, via the channel 7, being the proton
channel, an adiabatic pulse p.sub.1 is applied which, in the case of a
so-called 90.degree. adiabatic pulse, brings an equilibrium proton
magnetization volume-selectively into a transverse state. This is shown in
FIG. 2B in a rotating proton coordinate system x', y' and z' with the
magnetization M. In the case of an adiabatic pulse, the modulator 11 in
the proton channel is an ampitude/frequency modulator. FIG. 2C shows the
variation of the RF field B.sub.1 in the case of an adiabatic pulse as a
function of time t with simultaneous variation of the frequency, varying
from .omega..sub.0 +.DELTA..omega. to .omega..sub.0, i.e. with a frequency
swing .DELTA..omega. about the proton resonant frequency .omega..sub.0.
Subsequently, via the proton channel 7 a spin-lock pulse p.sub.2 is
applied to the object 5 under the control of the programmed means 18. The
B.sub.1 field of the spin-lock pulse p.sub.2 should then be directed along
the same axis as the magnetization M. During the spin-lock pulse p.sub.2 a
cross-polarization transfer pulse p.sub.3 is applied to the object 5 via
the .sup.13 C channel 8. After termination of the pulses p.sub.2 and
p.sub.3, a resonance signal 20 is sampled in the proton channel 7 and
processed in the processing means 17, using Fourier transformation, so as
to obtain a spectrum. Due to the transfer a carbon magnetization is
observed which has been intensified by the ratio of the gyromagnetic
ratios of protons and the carbon isotope coupled thereto, i.e. by a "gain"
factor 4. In this respect it is important that the so-called Hartmann-Hahn
condition is satisfied, i.e. in the present example the B.sub.1 fields
should relate as 1:4.
FIG. 2D shows Zeeman splitting of energy levels in the steady field for
.sup.1 H and .sup.13 C, spin-up and spin-down also being diagrammatically
shown therein. The Hartmann-Hahn transfer condition implies that the
energy gradients should correspond, i.e. the spin-locked system can
transfer energy only subject to the Hartmann-Hahn condition. In that case
the transfer is "smooth", in this respect it is to be noted that a
perturbance term should be present in liquid. In the relevant example this
perturbance term is present in the form of a J-coupling between .sup.1 H
and .sup.13 C, the speed of transfer being proportional to this coupling
constant, i.e. the time required for the cross-polarization transfer is
inversely proportional to the J-coupling. Proton-carbon couplings are
comparatively great (125-170 Hz), so that cross-polarization transfer
times of from 4 to 7 ms suffice.
FIG. 2E shows the behavior of transferred transverse magnetization M as a
function of time t. It appears that the transfer is an oscillatory
reciprocal process. The magnetization transferred can be sampled at the
first maximum at the instant t=t.sub.1 =1/J. In an absolute sense the
magnetization decreases due to relaxation which is inherently present; in
the present example this is the relaxation of fat in a rotating system of
coordinates, T.sub.1.rho., amounting to about 300 ms. For example, when a
glycogen coupling is considered, the relaxation in the rotating system of
coordinates is about two orders of magnitude smaller than that of fat. In
that case signal acquisition should take place prior to the first
magnetization peak, as shown in FIG. 2E, in order to obtain an optimum
result. In a human object 5 glycogen occurs notably in muscular tissue and
the liver and can provide an indication as regards the energy conditions
in the object. A deviating glycogen concentration will be found in the
case of metabolic disorders.
FIG. 3 shows a second version of a pulse and gradient sequence sq2 in
accordance with the invention as a function of time t. Via the first
channel 7, for example the proton channel .sup.1 H, a localized spin-echo
sequence is applied which comprises a localized excitation pulse p.sub.4
having a gradient G.sub.z and also a localized inversion pulse p.sub.5
having a gradient G.sub.x. At the instant t=t.sub.1, at the maximum of the
spin-echo signal SE, a spin-lock pulse p.sub.6 is applied to the object 5,
a cross-polarization transfer pulse p.sub.7 being applied at the same time
via the channel 8, for example the .sup.13 C channel. As a result, a
transferred resonance signal FID localized in a bar is obtained. By
application of a phase encoding gradient G.sub.y immediately after
termination of the spin-lock, resonance signals are obtained from parts of
the bar, it being possible to reconstruct spectra therefrom by means of
the programmed means 18.
In other versions, a volume-selective pulse and gradient sequence such as
PRESS, ISIS or STEAM is first applied to the first type of nucleus, thus
bringing the magnetization volume-selectively out of equilibrium.
Subsequently, Hartmann-Hahn transfer to the second type of nucleus takes
place. In the case of a PRESS sequence as described in European Patent
Application No. 0 106 226 and a STEAM sequence as described in U.S. Pat.
No. 4,748,409, spin-locking is applied at the maximum of the
volume-selective echo resonance signal or the stimulated spin-echo signal,
respectively. The excitation pulses are then spatially selective and no
magnetic gradient is applied during the occurrence of the volume-selective
resonance signal. In the case of the ISIS sequence as described in Journal
of Magnetic Resonance 66, 283-294 (1986), volume-selective magnetization
is obtained by 8-fold application of pulse and gradient combinations. The
ISIS excitation pulse is then preferably an adiabatic 90.degree. pulse and
spin-locking is applied during occurrence of a FID signal due to the ISIS
excitation pulse.
Because of the reciprocal nature of the Hartmann-Hahn transfer, measurement
is also possible via the channel which is not the customary channel. In
the case of proton-carbon coupling, however, it is then necessary to
enrich the carbon in practice, for example by injection or oral
administration of glucose to an object 5 whose metabolism can then be
traced. Additional phase encoding gradients can then also be applied in
order to obtain magnetic resonance signals for a spectroscopic image, for
example after the spin-lock pulses and before signal acquisition. The
latter method could be an alternative to PET (Positron Emission
Tomography) which utilizes radioactive compounds.
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